1. Field of the Invention
This invention relates generally to motor control circuitry and, more specifically, to digital circuitry suitable for controlling motor winding currents in servo motor systems.
2. Discussion of the Related Art
Applications in which objects are moved or positioned under computer control, or according to commands produced by any sort of digital processing system, generally use motors coupled to a digital control system to effect that movcment or positioning. Appropriate combinations of digital control signals, driver circuits, and motors can provide highly accurate rotations or rotational velocities so that the motor systems are suitable not only to provide highly accurate positioning but also highly accurate and reproducible velocities. Driver circuits for motor systems receive digital control signals and produce driving signals, typically square waves, to be applied across the windings of the motor. The accuracy of these motor systems is obtained by the driver circuits using feedback from the motor as to the motor position and through the high degree of control of the amplitude and timing of current signals that can be supplied to the windings of the motor. Particularly useful motors for these digitally controlled motion or positioning applications include two or three phase, permanent magnet motors.
An example of a portion of a conventional motor and motor control system is illustrated in FIG. 1. The illustrated motor might, for example, be a two-phase permanent magnet synchronous motor indicated in the figure by a single, isolated winding of the two-phase motor. Although the following background discussion is made in terms of a two-phase motor system, aspects of the present invention are believed to provide advantages when applied to the operation of other types of permanent magnet motors and in particular when applied to three-phase motors. One winding or coil 10 of the motor is indicated in FIG. 1 and is modeled as including an inductance 12 and a resistance 14 in series with the inductor.
The illustrated motor and driving circuit has a configuration generally referred to as an "H-bridge" in which a half-bridge is provided on either end of the winding 10 so that both ends of the coil 10 are driven by a half-bridge. Each half-bridge can drive both positive and negative voltages using an upper switching element 16, 18 and a lower switching element 20, 22. In the illustrative example the switches 16-22 are MOS field effect transistors ("MOSFETs"), although other types of switches including bipolar transistors are sometimes used in the illustrated H-bridge driving circuit. Each of the switches has a diode connected across the terminals of the switch. The catch or commutating diodes 24, 26, 28, 30 provide a parallel current path for the corresponding ones of the switches to catch the inductive spike output from the coil 10 that occurs when one of the switches of the half-bridge opens prior to the opposing switch closing.
One mode of operation that illustrates a function of the catch diodes can be shown beginning with the FIG. 1 H-bridge in a state with switches 16 and 22 switched on so that current flows from right to left through the coil 10. When switch 16 is switched off, the current flowing through the coil starts to decrease, reducing the magnetic flux within the coil. As this begins, the voltage at the junction indicated by 32 in FIG. 1 begins dropping rapidly. When the voltage at junction 32 falls to the point that diode 28 is forward biased, current begins flowing from ground through the catch diode 28, through the motor winding 10, through switch 22, and back to ground. After a period of dead time of the driver circuitry, switch 18 is turned on. The comparatively low on-state voltage of switch 18 in its on state causes the current flowing through the motor winding 10 to flow through the switch 18 and turns the diode 28 off. The current flowing from ground, through the lower half of the bridge (switch 20 or diode 28), through the motor winding 10, back through the lower right half of the driver (switch 22 or diode 30), and back to ground is referred to as "recirculating," since the current flows in a circle. In a recirculating mode of operation, transistors 16 and 18 are switched on while transistors 20 and 22 are off, or the inverse is true. It should be noted that the current does not recirculate indefinitely in these embodiments. The resistance of the driver and winding causes the magnitude of the current to gradually decay, although this is generally slow with respect to the chopping frequency.
There are a variety of different prior art methods for controlling the current through the motor winding 10. Typically these method rely on a measurement of the current passing through the motor winding to provide feedback on the operation of the motor. The measurement of the current through the winding may be made either directly through the winding itself or as the current flows through one or more of the switches (16-22 of FIG. 1). Methods for measuring the current include the use of current sensing resistors, magnetic field sensors, and current transformers at the switching element. For the purposes of illustrating the background of the present invention, the following discussion will emphasize switched mode current control.
Switched mode current controllers include those that use peak-sensing with recirculation, those that use peak sensing without recirculation, and others that use anti-phase methods. The drivers that use peak sensing with recirculation to control the motor current apply the supply voltage to the winding until the sensed winding current exceeds the desired threshold. The winding is then effectively short-circuited until it is time for the next power addition cycle. The next power additional cycle may be initiated following a fixed off time, as is characteristic of a variable chopping frequency drive, or at a fixed repetition rate for a fixed chopping frequency drive. These types of drives are effective for putting power into the motor winding, but arc not effective in removing the power. Further discussion of these drivers and their problems are provided in U.S. Pat. No. 5,710,499 to Carvajal, "Step Motor Control Circuit and Method." As stated in U.S. Pat. No. 5,710,499, drivers that use peak sensing with recirculation current control are known to reduce power output of stepper motors at higher speeds.
Conventional control methods, including the method of U.S. Pat. No. 5,710,499, are also not effective in decclerating loads in which the motor is primarily acting as a generator. It may be desirable to use a stepper motor as a generator to brake and slow the motor, significantly improving the ability of the motor to be stopped.
A further drawback of the fixed frequency recirculating control method is that the method provides very poor control of the motor when the motor winding current is near zero. The minimum on time of the switch circuit divided by the fixed chopping period sets a minimum duty cycle at which the driver may operate, as reflected in Equation 1. The minimum on time may result in significant minimum currents, which in turn may result in large crossover distortion as the current switches from a positive to a negative value within the bridge driving circuit. This in turn may not allow for accurate position or torque control of the motor attached to the control circuit. EQU V.sub.Minimum=V.sub.Supply* T.sub.Minimum/T.sub.cycle (1)
As discussed above, various designs using a recirculation mode of operation have drawbacks which reduce their desirability. The recirculation mode of operation, on the other hand, does minimize the magnetic losses of the motor by minimizing the Root Mean Square (RMS) amplitude of the applied voltage waveform. A low RMS voltage is achieved for the circuit because the winding voltage (i.e., the voltage across the motor winding) is approximately zero volts except for when power is being added to the motor winding. For the same reason, the recirculation modes of operation also limit the ripple current in the motor windings.
Drivers that use a peak sensing, non-recirculating mode of operation apply the power supply voltage to the winding until the desired current level has been exceeded, and then switch off all switches in the bridge, allowing the current to flow through the catch diodes until the following cycle. This mode of operation is capable of rapidly transferring power from the motor windings back to the power supply rail, and of controlling currents near zero, but it still has significant crossover distortion as the current changes from positive to negative within the bridge. The peak-sensing, non-recirculating mode of operation also produces significant motor heating due to the large AC component of the applied winding voltages and the large winding ripple currents, and the peak controlled current is often significantly different from the average current desired within the motor winding.
Yet another type of motor current control is the anti-phase method. The anti-phase method of control switches only between the forward voltage (switches 16, 22 on) and reverse voltage (switches 18, 20 on) during operation. The IXMS-150 manufactured by IXYS Corporation is a good example of a part that operates in this mode. Anti-phase operation provides excellent four-quadrant control of current, delivering power to the motor in a desirable manner and desirably facilitating use of the motor as a generator when decelerating a load. The anti-phase mode of operation exhibits good control of motor winding currents near zero while maintaining fairly low cross over distortion. On the other hand, motors operating in accordance with an anti-phase mode of operation may suffer from high ripple currents and magnetic losses in the motor, which tend to heat the motor. Other disadvantages of the anti-phase mode of operation include the fact that the current control loop must be tuned for the particular motor windings, and the requirement of current sensing.
Of the three control modes discussed, motors operating according to the anti-phase method exhibit the lowest crossover distortion. The driver stage illustrated in FIG. 1 generates a significant cross over distortion due to the dead-times associated with the finite switching times of the switches. The effect will be discussed in the following for a full switching cycle (high to low and back to high) for both possible current directions.
Referring again to FIG. 1, the first analysis is taken with current flowing through the motor winding 10 from left to right, with emphasis on the half bridge consisting of switches 16, 20 and diodes 24, 28 driving the motor winding 10. At the beginning of this analysis, the upper switch 16 is conducting for an active high period. When this period ends, switch 16 is switched off. Current continues to flow left to right through the inductor 12, necessarily flowing up through diode 28, since diode 24 is reverse biased and non-conductive. After the crossover delay, switch 20 begins conduction and diode 28 switches off. After the lower switch 20 has conducted for the commanded low period, it is switched off. With the current flowing from left to right, then diode 28 must begin conducting to carry the current. After the dead time period, the upper switch 16 begins conduction and the lower diode 28 stops conducting. In this case, the low period of the left half-H bridge (16, 20, 24, 28) as seen by the motor winding 10 is extended by the cross over delay beyond the period dictated by the controlling pulse width.
The second analysis with current flowing from right to left will show that the upper diode 24 will conduct during the two dead-time periods, enhancing high period of the output voltage. The direction of the enhancement subtracts from the nominal duty cycle of the driver causing significant crossover distortion. In conventional systems, this distortion may be reduced by use of negative feedback in the current control loop, necessitating current monitoring in conventional motor control systems. In such conventional systems, the feed-forward portion of the control loop does not handle this cross over distortion.
Conventional methods of motor control systems thus suffer from high cost, complexity, and/or heat production caused by the need to measure the current. Conventional methods may further be undesirable in that they do not simultaneously have good four quadrant current control while limiting ripple current and magnetic losses in the motor.